1. Field of the Invention
The present invention relates to a method for detecting a position which is indicated on an information equipment or a large display panel by an electronic pen for entering characters and graphics, and in particular to a method for detecting an indicated position on a display device, in which a gray scale image is displayed by utilizing a drive waveform because the display device itself does not have a function of gray scale image display or cannot perform the gray scale image display without difficulty.
2. Description of the Background Art
A conventional method for directly entering characters, which are written on a display panel such as a gas panel or an LCD (Liquid-Crystal Display) panel, includes a method of employing a display panel and a tablet panel (pen input panel) in a stacked form, and a method of using a display panel also as a tablet panel. In the former method, as shown in FIG. 1, a tablet panel 112 provided with a tablet drive circuit 111 of, e.g., a pressure-sensitive type or an electrostatic induction type is stacked on a display panel 114 provided with a display drive circuit 113, and an electronic pen 115 is used for input. This method has been employed in various information equipments. However, this method suffers from a problem that it is difficult to reduce the size, weight or cost of the device.
The latter method is achieved by the display-integrated tablet shown in FIG. 2. In this tablet, a display drive circuit and its scanning electrodes are used also as a tablet drive circuit and detection electrodes for input with electronic pen 115. For example, a gas panel is disclosed in Japanese Patent Laying-Open No. 52-2335 (1977) and an LCD panel is disclosed in Japanese Patent Laying-Open No. 5-53726. In these panels, a coordinate is detected by utilizing a capacity coupling between a detection electrode at the tip end of electronic pen 115 and display electrodes of the display panel which are formed of a plurality of row electrode groups and a plurality of column electrode groups perpendicular to the row electrode groups.
The gas panel proposed in the above publication utilizes an electrostatic coupling between the display electrodes and the detection electrode at the tip end of the pen. In this panel, a scanning voltage is successively applied to the display electrodes one by one, and the coordinate of the electronic pen is determined by measuring a timing at which a voltage induced at the detection electrode of the electronic pen indicates a peak value.
According to the LCD panel proposed in the above publication, as shown in FIG. 3, one frame period is timedivided into a "display period" and a "coordinate detection period", and display and coordinate detection are performed in these periods, respectively. According to this panel, a scanning voltage is successively applied to the display electrodes during the coordinate detection period, and X and Y coordinates of the electronic pen are detected by measuring the voltages induced at the detection electrode of the electronic pen.
In recent years, a PDP (Plasma Display Panel) has received attention as a gas panel display device because its size can be increased. For adding the function of detecting the coordinate with the electronic pen to the PDP, one can conceive of using the display panel also as the tablet panel for reduction in size, weight and cost, as shown in FIG. 2.
However, according to detection of the coordinate of the electronic pen, a voltage induced at the detection electrode is low and has a slowly changing waveform to be detected, and an S/N is extremely low, as also described in the foregoing Japanese Patent Laying-Open No. 52-2335. This is because the display electrodes are successively scanned one by one for detecting the coordinate, and an electrostatic capacity between the electronic pen and the detection electrode is small. As a result, the S/N of the detected voltage is low, and therefore the detection accuracy is low.
According to the aforementioned publication No. 52-2335, therefore, a plurality of (e.g., three) detection electrodes in a divided form are employed, and signal voltages are produced by differential amplification or the like of voltages induced at the electrodes for improving the detection accuracy, respectively. However, the above structure complicates the structure of the electronic pen, resulting in increase in cost. Further, the electronic pen has a tip end of a large diameter, so that manual entry such as entry at an intended coordinate point with the pen cannot be performed easily.
Recently, display of not only a monochrome image but also an image including gray scale has been strongly desired.
However, the PDP itself cannot essentially display gray scale. Technology for displaying gray scale on the PDP has been developed. This display technology typically performs the display by dividing a display period for one frame into a plurality of sub-fields so as to display gray scale. This method, however, employs two coordinate detection periods, i.e., an X coordinate detection period and a Y coordinate detection period as shown in FIG. 3. This increase in number of the sub-fields reduces the display period and therefore each sub-field period decrease because the frame period is constant. This results in increase in a drive frequency of a drive circuit, so that the display control becomes difficult. In order to avoid this disadvantage, the sub-fields may be reduced in number. In this case, however, levels of the gray scale decrease in number, which reduces the display quality.